1. Field of the Invention
The present invention relates to a brazing method which produces a braze joint having excellent corrosion and oxidation resistances, and also relates to a brazed structure produced by the brazing method.
2. Description of the Background Art
With a globally growing interest in environmental issues, there has recently been an increasing demand for cleaning diesel engine emission gas. For the emission gas cleaning, an attempt has been made to suppress generation of NOx by EGR (emission gas recycling) in which a portion of the emission gas is passed through a heat exchanger for reducing the temperature of the emission gas and introduced into an engine for reducing the oxygen concentration of the intake gas, and heat generated by combustion is absorbed by the emission gas having a higher specific heat to reduce the combustion temperature.
The heat exchanger is produced by brazing members which are made of such material as a stainless steel and are to be joined to each other with the intervention of a braze joint formed by fusing and solidifying a brazing material. A copper brazing material having a melting point of not lower than 1,000° C. and having excellent corrosion resistance is typically used as the brazing material.
A clad material has recently been proposed as a material for the members to be joined for improving the corrosion resistance of the braze joint. As disclosed in JA-3350667-B, the clad material includes a base plate composed of a ferrous material and a Fe atom diffusion suppression layer laminated on the base plate and composed of pure Ni or a Ni-based alloy mainly containing Ni for preventing Fe atoms, which may deteriorate the corrosion resistance, from diffusing into the braze joint from the ferrous material. Further, JA-2003-145290-A proposes a Fe atom diffusion suppressing layer composed of a N—Cr alloy containing Cr in a proportion of not smaller than 10 mass % and not greater than 30 mass % to provide a braze joint of a Cu—N—Cr alloy for improvement of the oxidation resistance and the corrosion resistance of the braze joint.
As described above, the use of the clad material including the Fe atom diffusion suppressing layer improves the corrosion and oxidation resistances of the braze joint of the heat exchanger. In recent years, however, the emission gas cleaning has been more heavily demanded. Correspondingly, the corrosion resistance of the braze joint against emission gas condensate has been more heavily demanded.